Extends functionality of gather by allowing varying
counts of data from each process.
Java binding of the MPI operation MPI_GATHERV.
The sizes of arrays recvcounts and displs should be the
size of the group. Entry of displs specifies the
displacement relative to element recvoffset of recvbuf
at which to place incoming data. Note that if recvtype is a
derived data type, elements of displs are in units of the derived
type extent, (unlike recvoffset, which is a direct index into the
buffer array).